1. Field of the Invention
The present invention relates to dielectric ceramic compositions, and particularly to dielectric ceramic compositions used as materials of a ceramic laminated capacitor and the like.
2. Description of the Prior Art
Conventionally, as dielectric ceramic compositions having a small voltage dependency, a high ceramic strength and a flat dielectric temperature characteristic, those composed of, for example, BaTiO.sub.3 as a main component, to which bismuth compounds such as Bi.sub.2 O.sub.3 -TiO.sub.2 -SnO.sub.2, Bi.sub.2 O.sub.3 -ZrO.sub.2 and rare earth elements are added as by-components, are widely known.
While, separate from the dielectric ceramic compositions having the above-mentioned composition, those composed of BaTiO.sub.3 as a main component, to which Nb.sub.2 O.sub.5, rare earth oxides and transition metal oxides of Cr, Mn, Fe, Co, Ni are added as by-components, have been proposed as the dielectric ceramic composition which has a flat dielectric temperature characteristic, irrespective of its high dielectric constant of 3000 or more.
Temperature characteristics of these dielectric ceramic compositions is that, X7R characteristics of the EIA standards is satisfied, or a capacitive change factor based on an electrostatic capacity at +25.degree. C. in a temperature range of -55.degree. C. to +125.degree. C. is within .+-.15%.
In recent years, ceramic laminated capacitors are used in ECC modules (electronic control devices of an engine) installed in an engine soon of an automobile. Since the device is designed for stable control of the engine, viewing from the temperature stability of circuitries, it is desirable to satisfy the R characteristics (capacitive change factor within .+-.15%) as the temperature characteristics of the capacitors being used.
While, in winter in cold districts, temperature goes down to around -20.degree. C. in the engine room of the automobile, and in summer, the temperature rises to around +130.degree. C. when the engine is started. Particularly, when the engine is over heated, the temperature easily goes up to around 150.degree. C. Thus, the conventional dielectric ceramic compositions which satisfy the X7R characteristics can not cope with the high engine room temperature any longer.
Since the laminated capacitors are for use in the automobile, when it breaks, while being mounted on the circuit board, the ECC modules can not function sufficiently, and in the worst case, there is the possibility of causing an accident, which must be avoided by all means. Furthermore, since vibration and stress are always exerted while the automobile is running, in order to avoid damages due to such vibration and stress, the ceramics must be sufficiently strong.
When the voltage dependency of the dielectric ceramic composition is large, the dielectric can not be processed into a thin film, thus the small-sized and large capacity ceramic laminated capacitor can not be manufactured, besides, it is also not preferable from a viewpoint of circuit stability.
Now, in some cases, the dielectric ceramic composition composed of BaTiO.sub.3 as a main component, to which Nb.sub.2 O.sub.5, rare earth compounds and transition metal oxides of Cr, Mn, Fe, Co and Ni are added as by-components, was problematic in that, it is apt to break at the time of mounting to the circuit board because the ceramic strength is weak.
Since the voltage dependency of the dielectric ceramic composition having a large dielectric constant is large, it can not meet with the recent thin-film technique, thus the small-sized and large capacity ceramic laminated capacitor could not be manufactured.
Meanwhile, as stated above, though the dielectric ceramic compositions composed of BaTiO.sub.3 as the main component, to which bismuth compounds are added as the by-components, have a small voltage dependency and a high ceramic strength, when the dielectric constant is raised, a temperature change factor of the dielectric constant becomes larger. Also, in the case of ceramic laminated capacitor, when a baking temperature is raised to 1160.degree. C. of higher , an internal electrode must contain Pb by 30 wt. % or more. Hence, Pb and Bi.sub.2 O.sub.3 in the internal electrode apt to react one another, furthermore, a cost of the internal electrode increases.